Day 1

.envrc

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+watch_file shell.nix
+use nix shell.nix

.gitignore

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+target/
+tags
+.direnv

1/Cargo.lock

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+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "aoc-2020-1"
+version = "0.1.0"

1/Cargo.toml

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+[package]
+name = "aoc-2020-1"
+version = "0.1.0"
+authors = ["Benjamin Bädorf <hello@benjaminbaedorf.eu>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

1/src/main.rs

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+use std::fs;
+
+fn main() {
+    // part_1();
+    part_2();
+}
+
+fn part_1() {
+    let contents = fs::read_to_string("./input").expect("Couldn't read file!");
+    let numbers: Vec<usize> = contents
+        .lines()
+        .map(|s| s.parse::<usize>().unwrap())
+        .collect();
+
+    let initial_condition: (usize, usize) = (0, 1000000);
+
+    let increases = numbers.iter().fold(initial_condition, |acc, num| {
+        if num > &acc.1 {
+            (acc.0 + 1, *num)
+        } else {
+            (acc.0, *num)
+        }
+    });
+
+    println!("{}", increases.0);
+}
+
+fn part_2() {
+    let contents = fs::read_to_string("./input").expect("Couldn't read file!");
+    let numbers: Vec<usize> = contents
+        .lines()
+        .map(|s| s.parse::<usize>().unwrap())
+        .collect();
+
+    //                      inc     n - 3  n - 2  n - 1
+    let initial_condition: (usize, (usize, usize, usize)) = (0, (0, 0, 0));
+
+    let increases = numbers.iter().fold(initial_condition, |acc, num| {
+        let new_previous_numbers = (acc.1 .1, acc.1 .2, *num);
+        if acc.1 .0 == 0 {
+            return (0, new_previous_numbers);
+        }
+
+        let previous_sum = acc.1 .0 + acc.1 .1 + acc.1 .2;
+        let current_sum = acc.1 .1 + acc.1 .2 + num;
+        if current_sum > previous_sum {
+            (acc.0 + 1, new_previous_numbers)
+        } else {
+            (acc.0, new_previous_numbers)
+        }
+    });
+
+    println!("{}", increases.0);
+}

shell.nix

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+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell {
+  buildInputs = [
+    pkgs.rustup
+  ];
+}